The desired properties of tissue, including softness, bulk, stretch and absorbency, are typically achieved by the means of a steam-heated Yankee dryer cylinder and a doctor blade. A wet fiber web is largely dewatered at the pressure roll nip. At this point, the sheet is transferred to the Yankee surface and has a consistency of about 35-80%. The sheet is further dried on the hot Yankee cylinder to a typical consistency of 90-98% and removed with a doctor blade. The mechanical action of the blade results in a disruption of the fiber-fiber bonds and a formation of a microfold structure within the sheet. This process is referred to as creping.
In order to develop the crepe, the paper web has to adhere to the surface of the Yankee dryer. The adhesion is provided by applying an adhesive to the dryer. Most common Yankee adhesives are synthetic polymers such as polyaminoamides, polyamides, polyamines, polyvinyl alcohols, polyvinyl acetates, polyacrylamides and polyethers. Various low molecular weight compounds are used to modify the Yankee coating properties.
A superior Yankee coating should form a film that provides good adhesion for efficient creping, and is also uniform, durable and soft. Uniformity of the coating is critical not only for consistent creping that affects the quality of the produced sheet, but also for even coverage of the Yankee surface to prevent premature wear of the cylinder and the blade. Durability is a characteristic of how stable the coating is on the Yankee surface, particularly at the pressure roll nip. If the coating is easily washed off, it does not protect the Yankee cylinder and leads to excessive creping blade wear. A hard coating causes blade chatter, which results in blade wear and non-uniform creping. A soft, but durable coating is preferred.
Polyaminoamides crosslinked with epichlorohydrin (PAE) are the most widely used adhesives in commercial practice by a wide margin. Depending on the grade and creping conditions, adhesion, durability and softness of the coating can be adjusted to an appropriate level by using combinations of PAE resins having various molecular weight and/or modifiers such as release oils, surfactants, humectants, phosphates, etc. Yet, the applicability of PAE resins is not universal and they do not always work for all tissue grades and conditions. The most common problem that is faced with PAE adhesives is their tendency to build up on the Yankee dryer, sometimes referred to as “hard” coating. This results in blade chatter causing picks or holes in the sheet and resulting in sheet breaks and machine downtime. Moreover, even though the amounts of epi and epi by-products are very low in the modern PAE adhesives, the industry would prefer to use non-epi adhesives if an alternative existed. However, other non-epi alternatives including polyvinyl alcohols, polyvinyl acetates, polyacrylamides and polyethers, also have deficiencies regarding the coating properties they provide in that there is not an optimum combination of coating durability, softness, adhesion and uniformity. Accordingly, there is an ongoing need for improved creping adhesive compositions.